Commitments, contingencies and litigation (Details) (USD $)	0 Months Ended	3 Months Ended
	Aug. 06, 2012	Jun. 30, 2012	Sep. 30, 2010	Dec. 31, 2014	Dec. 31, 2013	Aug. 31, 2011	Oct. 31, 2010
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters				$ 27,600,000	$ 29,500,000
Litigation related to construction materials [Member]
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss						3,700,000
Additional amount per day of potential penalties						5,000
Former employee litigation [Member]
Loss Contingencies [Line Items]
Plaintiffs request compensatory damages	23,800,000
Natural gas gathering operations [Member]
Loss Contingencies [Line Items]
Arbitration award							26,600,000
Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, before tax		(24,100,000)	26,600,000
Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, after tax		$ (15,000,000)	$ 16,500,000